1. Field Of The Invention
This invention relates to the generation of signals in the terahertz frequency range and the detection thereof.
2. Prior Art
An aspect of computer cycle speed is the time necessary for communication between logic circuitry and/or memory circuits. By increasing packing density and reducing the physical space between such elements, processing speeds may be increased by decreasing the signal path length. There are limits on packing density given problems of signal crosstalk, heating and the like. Thus other techniques for increasing system speed are needed once the option of increasing device packing density has been fully utilized. A corresponding requirement to attain higher processing speeds is the need for timing circuits which operate at very high frequencies yet provide stable clock pulses, uniformly distributed in time.
Early attempts to generate subpicosecond signals are reported in Auston, "Subpicosecond Electro-Optical Shock Waves", Appl. Phys. Lett. 43 (B) October 1983, pp. 713-715 and Auston et al. "Cherenkov Radiation from Femtosecond Optical Pulses in Electro-Optic Media", Phys. Rev. Lett. 53 1555, October 1984.
Reference is made to M. B. Ketchen, et al, "Generation of Subpicosecond Electrical Pulses on Coplaner Transmission Lines", Appl. Phys. Lett. 48(12), 24 March 1986, pp. 751-753. This publication describes techniques to generate ultrashort electrical pulses by photoconductively shorting charged transmission lines and narrow gaps. As reported in that article, a transmission line having a design impedance of 100.OMEGA. was made using three parallel 5-.mu.m wide aluminum lines which were separated from each other by 10-.mu.m. That transmission line together with its contact pads was fabricated on an undoped silicon on sapphire (SOS) wafer, which was subsequently ion implanted to shorten the carrier lifetime. The transmission line was photoconductively shorted utilizing 80 fs laser pulses. The exciting beam had a spot diameter of 10 .mu.m which bridged two of the three parallel transmission lines in a "sliding contact" arrangement. By utilizing a sampling beam coupled to a multichannel analyzer, subpicosecond electrical pulses were measured having an actual pulse width less than 0.6 psec.
U.S. Pat. No. 4,251,130 defines a pulse generator using an optical gate to generate subpicosecond pulses. Pulses are created by having light pass back and forth between the facets of a waveguide in synchronism with a bias signal. The coupling characteristic is controlled along an optical direction coupler by the narrow band bias signal such that a zero coupling occurs only in the region about the zero points of the travelling bias signal.
U.S. Pat. No. 4,372,643 describes an ultrafast gate employing a transmission line with a standing wave set-up along it. The '643 patent employs a resonant electric signal circuit to generate large amplitude signals while consuming relatively little signal power.
While not prior art to this invention, DeFonzo et al in "Transient Response of Planar Integrated Optoelectronic Antennas", Appl. Phys. Lett. 50 April 1987, pp. 1155-1157 and "Optoelectric Transmission and Reception of Ultrashort Electrical Pulses", Appl. Phys. Lett. July 1987, pp. 212-214, reports the use of planar antenna structure fabricated on a radiation-damaged silicon-on-sapphire substrate for the detection of picosecond pulses.
Also, reference is made to Smith et al. "Subpicosecond Photoconducting Dipole Antennas" IEEE J Quantum Elect. 24, February 1988 No. 2, pp. 255-260 which, while not prior art to this invention. discusses the generation of subpicosecond electrical pulses, the use of small dipoles for transmission and coherent detection.